Led-based replacement for fluorescent light source

ABSTRACT

A light source that is adapted to replace existing fluorescent tubes in an existing fluorescent light fixture is disclosed. The light source includes a plurality of LEDs mounted on a heat-dissipating structure, first and second plug adapters that mate with the florescent tube connectors of the fluorescent tube the light source is to replace, and a power adapter that converts power from a fluorescent tube ballast presented on the first and second plug adapters to DC power that powers the LEDs. The light source is powered from the output of the existing fluorescent ballast. The light source can utilize the existing metallic enclosure as a heat-radiating surface and/or direct air heat transfer from the surface of the heat-dissipating structure.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are an important class of solid-statedevices that convert electric energy to light. Improvements in thesedevices have resulted in their use in light fixtures designed to replaceconventional incandescent and fluorescent light sources. The LEDs havesignificantly longer lifetimes than both incandescent bulbs andfluorescent tubes. In addition, the efficiency of conversion ofelectricity to light has now reached the same levels as obtained influorescent light fixtures.

In many applications, the cost of replacing a fluorescent tube anddisposing of the defective tube is much greater than the cost of thetube itself. In commercial settings, the labor cost inherent in stockingand replacing tubes is high. In addition, the cost of disposal offluorescent tubes further increases the cost of replacing the tubes,since fluorescent tubes utilize mercury to generate the underlying UVlight that is then converted to visible light by the phosphors in thetubes, which are also hazardous materials that present a separate healthhazard. Hence, care must be taken in handling the tubes and moving themto the disposal site to prevent breakage and the subsequent release ofthe toxic materials. Finally, the cost of the tubes themselves over thelifetime of the light fixture is also significant. Thus, there has beensome interest in replacing existing fluorescent tubes with LED-basedlighting elements.

Converting existing fluorescent fixtures to LED-based light sourcespresents a number of challenges. Existing fluorescent fixtures typicallyinclude an enclosure that holds one or more tubes and a ballast thatconverts the 50 or 60 cycle building power to the voltages andfrequencies utilized by the tubes. A fluorescent tube typically has astartup phase in which a discharge is initiated in the gas. Once thedischarge is established, a different voltage is applied to thefluorescent tube to maintain the discharge. The ballast provides thedriving voltages and manages the startup phase. The voltages used tostart and drive the fluorescent tube are typically AC voltages.

LEDs are typically driven with a DC voltage at a constant current.Hence, an LED light source typically requires a power supply thatconverts the conventional building power to a DC source that supplies aconstant current to the LEDs in the light source. Accordingly, when anexisting fluorescent tube light source is converted to an LED-basedlight source, all of the fluorescent tubes are typically removed and thefluorescent tube ballast is replaced with a LED power source.

This conversion can impose a significant cost and require a significantdowntime for the lighting fixtures. The retired fluorescent tubes thatare still functioning represent a significant capital investment that islost when the fixture is replaced. In addition, specialized personnelare needed to remove the old ballast and install the new LED powersupply. Accordingly, a switchover to LED-based tube replacements canrequire a significant level of organization and planning as well ascost.

The electrical conversion efficiency and lifetime of LEDs depend on theoperating temperature of the LEDs. Increases in temperature lead to aloss in conversion efficiency and a lowering of the LED lifetime.Transferring the heat from the LEDs to the surrounding air is, hence, animportant consideration in replacing existing fluorescent tubes withLED-based light sources. Typically, LED light sources in the power rangeof existing fluorescent tube light sources require a large heat transfersurface and good air circulation. If the fluorescent tubes are in aclosed fixture that has poor air circulation, the transfer of the heatto the air presents additional challenges.

SUMMARY OF THE INVENTION

The present invention includes a light source that is adapted to replaceexisting fluorescent tubes in an existing fluorescent light fixture. Thelight source includes a plurality of LEDs mounted on a heat-dissipatingstructure, first and second plug adapters that mate with the florescenttube connectors of the fluorescent tube the light source is to replace,and a power adapter that converts power from a fluorescent tube ballastpresented on the first and second plug adapters to DC power that powersthe LEDs. The light source is powered from the output of the existingfluorescent ballast. The light source can utilize the existing metallicenclosure as a heat-radiating surface and/or direct air heat transferfrom the surface of the heat-dissipating structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view from underneath a typical prior art fluorescent tubelighting assembly, and FIG. 1B is a cross-sectional view of lightingassembly 20 through line 1B-1B.

FIGS. 2 and 3 illustrate one embodiment of an LED-based light sourcethat is configured as a replacement for a conventional fluorescent tube.

FIG. 4 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention mounted against the back wall ofenclosure 21 shown in FIGS. 1A and 1B.

FIG. 5 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention.

FIG. 6 illustrates one embodiment of a power adapter according to thepresent invention.

FIG. 7 illustrates another embodiment of a power adapter according tothe present invention.

FIG. 8 is a cross-sectional view of another embodiment of a light sourceaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIGS. 1A and 1B. FIG. 1A is aview from underneath a typical prior art fluorescent tube lightingassembly, and FIG. 1B is a cross-sectional view of lighting assembly 20through line 1B-1B. Light source 20 includes an enclosure 21 that housestwo fluorescent tubes shown at 22 and a ballast 24 for powering thefluorescent tubes. Light source 20 also includes a diffuser plate 26that is not shown in FIG. 1A. The fluorescent tubes are inserted intoconnectors 23 that are connected to ballast 24 and provide the power foroperating the fluorescent tubes.

The specific electrical connections and type of connector utilizeddepends on the type of fluorescent tube for which light source 20 isdesigned. For example, some designs utilize fluorescent tubes that havefilaments within the tubes that are used for starting the discharge inthe tube; while other designs utilize a high voltage startup phase tostart the discharge. The first type of fluorescent tube utilizes aconnector that mates with two pins on each end of the fluorescent tube.The second type of fluorescent tube has only one pin on each end of thefluorescent tube. The details of the ballast operation will be discussedin more detail below. For the purposes of the present discussion, it issufficient to note that ballast 24 typically provides a first powerpattern used to start the fluorescent tube and a second power patternused to maintain the discharge during operation.

Conversion systems for converting fluorescent tube lighting to LEDlighting are available commercially. In one scheme, the ballast isreplaced by a power supply for providing a constant current source tothe LEDs. This procedure requires the conversion to be carried out bysomeone with significantly more expertise than that required to replacea burnt-out fluorescent tube. In addition, the investment in the ballastis lost. In this regard, it should be noted that some of the circuitryin the ballast could, in principle, be utilized to simplify thecircuitry needed to construct the constant current source for the LEDs.

Commercial lighting elements based on LEDs that emulate a conventionalfluorescent tube are also available. These replacement tubes plug intothe same connectors as the fluorescent tubes that they replace after theballast has been replaced or the ballast has been removed and theconnectors wired directly to the AC power lines. Since thesereplacements have the same form factor as the fluorescent tubes thatthey replace, heat dissipation presents challenges that significantlyincrease the cost of the replacements.

The present invention is based on the observation that a replacement LEDlight source for fluorescent tube 22 should plug into the sameconnectors and operate from the voltage patterns generated by theballasts. Such a replacement would permit the conversion fromfluorescent tubes to LED light sources to take place gradually as thefluorescent tubes burn out and are replaced. Furthermore, the upgradecould be carried out by the same personnel who are presently employed tochange fluorescent tubes when those fluorescent tubes burn out. Inaddition, as will be discussed in more detail below, the power supply onthe LED light source can advantageously use the output waveforms of theballast for a number of different types of ballasts that are currentlyin use.

Refer now to FIGS. 2 and 3, which illustrate one embodiment of anLED-based light source that is configured as a replacement for aconventional fluorescent tube. FIG. 2 is a top view of light source 30,and FIG. 3 is a cross-sectional view of light source 30 through line3-3. Light source 30 is constructed from a plurality of LEDs of whichLED 31 is typical. In this embodiment, the dies emit blue light and arecovered with a layer of phosphor 32 that converts a portion of the lightgenerated by the corresponding die to light in the yellow region of thespectrum such that the light that leaves the phosphor layer is perceivedas being white light of predetermined color temperature by a humanobserver. In this embodiment, the LEDs are connected in parallelutilizing traces 34 and 35, which are formed on an insulating layer 33.However, embodiments in which the LEDs are connected in series or morecomplex circuit patterns can also be accommodated. Each LED is alsomounted on a heat-dissipating structure 36 that aids in the transfer ofthe heat generated by the LEDs to the ambient environment. The manner inwhich the heat is dissipated will be discussed in more detail below.

Traces 33 and 34 are powered from an interface circuit having componentsshown at 37 and 38. The interface circuit is connected to the connectorsused to power the existing fluorescent tube via cables 40 and 43 thathave connectors shown at 42 and 41, respectively. Connectors 42 and 41have ends that match the ends of the fluorescent tube that light source30 replaces.

As noted above, heat dissipation is an important consideration with highpower LED light sources. A T12 or T8 fluorescent tube typicallygenerates 70 to 100 lumens per inch of tube while consumingapproximately 0.9 watts of power per inch of tube. Hence, an LEDreplacement light source needs to generate 70 to 100 lumens per inch oflight source. In addition, the LED replacement light source must have a“footprint” that is no wider than the fluorescent tube. Currently, whitelight sources based on GaN blue emitting LEDs generate 70 to 100 lumensper watt. Hence, the LED replacement source will generate about 1watt/sec of heat for each inch along the light source to provide areplacement for the higher efficiency fluorescent tubes. If a lowerefficiency fluorescent tube is being replaced, the heat that must bedissipated could be as low as 0.5 watts/sec. On the other hand, an LEDreplacement light source could provide more light than existingfluorescent tubes of the length being replaced, and hence, heatdissipations in excess of 2 watts/sec/inch of fluorescent tube could berequired.

This heat needs to be dissipated without raising the temperature of theLEDs beyond the point at which an unacceptable decrease in theelectrical conversion efficiency of the LEDs is incurred or atemperature at which the life of the LED is reduced. The operatingtemperature of the LEDs depends on the power being dissipated in theLEDs, the efficiency with which that heat is transferred to the ambientenvironment, and the temperature of the ambient environment. Typically,a maximum temperature of 70° C. is the operating limit; however, highertemperatures could be utilized with some LEDs, for example limits of100° C. or 150° C. could be utilized.

In one aspect of the present invention, the LEDs are distributed onheat-dissipating structure 36 such that the surface area presented bythe heat-dissipating structure is sufficient to dissipate a significantfraction of the heat of each LED to the surrounding air without the useof finned heat radiators or direct connections to larger heat-radiatingsurfaces. In one embodiment, the LEDs are distributed such that the heatgenerated on heat-dissipating structure 36 is less than 1 watt persquare inch of surface area on the top surface of heat-dissipatingstructure 36. As noted above, this arrangement is well matched toexisting fluorescent tubes. It should be noted that T12 fluorescenttubes have a diameter of 1.5 inches; hence, a heat-radiating surface ofabout 1.5 square inches per inch of fluorescent tube being replacedcould be made available. It has been determined experimentally, that ifthe LEDs are spaced apart along the length of heat-dissipating structure36, between 0.5 and 1 watt of heat per square inch of surface area canbe dissipated without raising the temperature of the LEDs by more than30° C. above ambient.

Referring to FIG. 3, if additional heat dissipation is required, anadditional heat-dissipating surface 47 on the backside of the lightsource can be utilized. In one aspect of the present invention, lightsource 30 includes a metallic layer 39 that is separated fromheat-dissipating structure 39 by insulating layer 33. In one aspect ofthe invention, a metal layer is connected to heat-dissipating structure36 by vertical heat-conducting areas such as the metal filled via shownat 45. In another aspect of the invention, heat-dissipating structure 36is part of metal layer 39. In this case, insulating layer 33 is absentfrom the area under heat-dissipating structure 36.

It should be noted that heat-dissipating structure 47 can transfer heatto the air if light source 30 is suspended above the back wall 25 ofenclosure 21 shown in FIGS. 1A and 1B. In this case, cables 40 and 41shown in FIGS. 2 and 3 could be replaced by rigid members such thatlight source 30 would be suspended within the enclosure in the samemanner as the fluorescent tube that it replaces.

However, more effective heat transfer can be provided by mounting lightsource 30 such that surface 47 is in thermal contact with the back wallof the existing fluorescent tube enclosure. Refer now to FIG. 4, whichis a cross-sectional view of another embodiment of a light sourceaccording to the present invention mounted against the back wall ofenclosure 21 shown in FIGS. 1A and 1B. Light source 50 utilizes aheat-dissipating structure 52 that is part of heat-dissipating structure59 and has a sufficient contact area with heat-dissipating structure 59to assure that the thermal resistance between heat-dissipating structure52 and heat-dissipating structure 59 is much less than the thermalresistance between the LEDs 51 and heat-dissipating structure 52.Heat-dissipating structure 59 has a surface 58 that is in thermalcontact with wall 25 of enclosure 21 when light source 50 is mounted inenclosure 21. The electrical contacts used to power the LEDs are onseparate traces on the surface of heat-dissipating structure 52. Thesetraces cover a portion of the surface of heat-dissipating structure 52;however, the fraction of the surface so covered is small compared to thearea of heat-dissipating structure 52 that is exposed to the air.

Light source 50 can be attached to surface 25 by a variety of methods.As noted above, one important goal of the present invention is toprovide a replacement light source that can be installed by the sameperson who would normally replace a defective fluorescent tube in thesame enclosure. For example, light source 50 could be bolted to surface25. However, attachment methods that require that the new light sourcebe bolted to enclosure 21 are not preferred, since such schemes canrequire that new holes be drilled in enclosure 21.

Light source 50 could also be supplied with a heat-conducting adhesivethat is applied prior to making the attachment of light source 50 tosurface 25. The adhesive is applied over a significant area, and hence,the thermal resistance of the adhesive is less of an issue. To simplifythe attachment, light source 50 could be supplied with the adhesivealready applied to surface 58 and protected by a peal-off strip. Whilethis method can be practiced by low-skilled personnel, the methodpresents problems if the light source must be removed at a future datedue to failure or the desire to further upgrade the light source. Inaddition, the coefficient of thermal expansion of the material fromwhich heat-dissipating structure 59 is constructed will, in general, bedifferent from that of the material from which enclosure 21 isconstructed; hence, the thermal cycling of the light source can weakenthe attachment bond over time leading to a bond failure or loss ofthermal conductivity when gaps form between surfaces 25 and 58.

In one aspect of the present invention, the attachment mechanism makesuse of the observation that existing enclosures are often constructedfrom steel. Hence, if heat-dissipating structure 59 is constructed froma material that is magnetized, such as iron, light source 50 can beattached magnetically to surface 25. In another aspect of the invention,a heat-conducting grease is applied to surface 59 to assure good thermalcontact between the two surfaces. This grease also facilitates themovement of light source 50 on surface 25 during the positioning oflight source 50 on the surface. Since the surface area of enclosure 21is much greater than the area available on the heat-dissipating surfaceof the replacement light source, considerably higher power dissipationlevels can be accommodated. As a result, a replacement light source thatis significantly brighter than the fluorescent tube being replaced canbe provided.

In one aspect of the present invention, the replacement LED light sourcehas a plurality of LEDs mounted on a heat-dissipating substrate that hasa width that is less than or equal to the diameter of the fluorescenttube that the light source is designed to replace. The substrates aremetal clad printed circuit boards in which the metal cladding providesthe heat-radiating surface. The LEDs are mounted in a spaced apartarrangement such that the heat generated on any 1 square inch section ofthe substrate is less than a predetermined design power value thatdepends on the manner in which the substrate dissipates the heat to thesurrounding environment. The goal of the LED placement is to providesufficient heat dissipation to limit the temperature of the LEDs to lessthan 75° C. above ambient.

Refer again to FIG. 2. For example, if the substrate has aheat-radiating surface that dissipates the heat to the surrounding airsuch as the surface of heat-dissipating structure 36, the presentinvention limits the LED density such that the heat generated on eachsquare inch of heat-dissipating surface is less than 2 watts andpreferably less than 1 watt; while accommodating LEDs that dissipate atleast 0.5 watts of heat per second. It should be noted that the relativesizes of heat-dissipating structure 36 and traces 33 and 34 are notshown to scale. In practice, the width of heat-dissipating structure 36is substantially equal to the width of the light source. Hence, if LEDshaving a power dissipation of 1.5 watts each are mounted at one inchspacing on heat-dissipating structure 36 and heat-dissipating structure36 is 1.5 inches wide, each LED will be surrounded by an area of 1.5square inches of heat-dissipating surface. This maintains the powerdensity at a level of 1 watt per square inch.

If the heat-dissipating surface is increased by utilizing the surface 47shown in FIG. 3 in addition to the surface of heat-dissipating structure36 then the density of LEDs can be increased further. For example, iflight source 30 were mounted such that air circulated on both sides oflight source 30, this additional surface area could be utilized todissipate heat thereby doubling the power density on the substrate.

Finally, if the arrangement shown in FIG. 4 is utilized, then theeffective heat-dissipating surface is still greater. The effective sizedepends on the thermal resistance between surfaces 25 and 58. In thisregard, it should be noted that enclosure 21 is often painted. The paintlayer limits the thermal conductance of the interface between surfaces25 and 58.

The above-described embodiments of the present invention have utilized adesign requirement that the temperature rise of the LEDs is held to lessthan 75° C. over ambient. However, embodiments in which the designcriterion is greater or less than this amount could also be utilized.For example, the replacement light sources could be constructed suchthat the maximum increase in temperature over ambient is 20° C., 30° C.,40° C., 50° C., 60° C., or 70° C.

Typically, the LED dies are of order 1 to 2 mm and dissipate between 1and 2 watts. In one aspect of the present invention, the dies aremounted directly on the heat-dissipating structure, as opposed tomounting packaged LED dies on the heat-dissipating structure. In oneaspect of the present invention, the dies are separated by one to twoinches on the light source and the width of the heat-radiating surfaceis chosen such that the area surrounding each die has sufficient surfacearea radiating heat to the environment to assure that the temperature ofthe die remains less than 75° C. above ambient when the die is powered.Even with the phosphor layer over the dies, the light source appears tobe discrete point emitters from the point of view of an observer in thearea illuminated by the final light source. Fluorescent tubes, incontrast, are broad linear sources. Hence, in some applications,additional optics may be necessary to give the appearance of afluorescent tube-like light source. Since the individual LEDs are oforder 2 or 3 mm, a cylindrical lens that is located one or two inchesabove the LEDs can transform the collection of point sources into alinear source that is collimated or that diverges at a predeterminedangle. Such an arrangement is shown in FIG. 5, which is across-sectional view of another embodiment of a light source accordingto the present invention. In light source 60 a cylindrical lens 61 ismounted above the LEDs. It should be noted that most fluorescent tubeenclosures have a depth of a few inches, since the enclosures mustaccommodate fluorescent tubes that are typically 1.5 inches in diameter.Hence, the LEDs appear as point sources at the lens.

Alternatively, the diffuser 26 shown in FIG. 1B could be replaced by adiffuser having a cylindrical lens stamped on the diffuser. Thisarrangement, however, requires the user to change the diffuser, andforecloses the user from changing a single fluorescent tube and leavingthe remaining tubes in place until they fail.

The above-described embodiments of the present invention utilize a poweradapter system that connects to the existing fluorescent tube connectorsin the light fixture and powers the LEDs on the light source. Consider alighting fixture that has 4 fluorescent tubes powered from a commonballast. This light fixture could be converted to LED-based lighting byreplacing all of the fluorescent tubes at once and changing the ballastto one that accepts AC and outputs DC of the desired voltage and currentto run the LEDs. However, as noted above, this operation requires askilled tradesman. Alternatively, the fluorescent tubes could bereplaced as they fail. Each time a fluorescent tube fails, theindividual who is normally responsible for replacing dead tubes wouldreplace the dead tube with a light source according to the presentinvention. When all of the fluorescent tubes have finally been replaced,the existing ballast could, in principle, also be replaced to providehigher overall power conversion efficiency.

In one aspect of the present invention, the power adapters shown at 37and 38 in FIG. 2 convert the output of the fluorescent tube ballast toDC having the desired voltage and current needed to drive the LED lightsource. The remaining fluorescent tubes in the enclosure still receivethe same power as before; hence, these fluorescent tubes do not need tobe replaced until they fail. The form of the power adapter depends tosome degree on the type of ballast that is employed in the light fixtureat the start of the conversion process. However, in general, there is astartup phase in which the ballast applies signals needed to initiatethe discharge in the tube and a maintenance phase in which the voltagepattern across the fluorescent tube is maintained at level that allowsthe discharge to continue. In one aspect of the present invention, thepower adapters present loads to the fluorescent tube ballast at startupthat mimic the current flows associated with the fluorescent tube havingstarted while protecting the LEDs from any high voltages used in thestartup phase. Once, the ballast switches to the maintenance voltage,the power adapters convert the AC voltage associated with this phase ofoperation to DC at the voltage and current required by the LED lightsource.

The AC output of conventional ballasts is either at the AC linefrequency or an AC signal at frequencies in excess of 20 KHz, andpreferably greater than 42 KHz. Older ballast designs utilize the linefrequency. More modern “digital” ballasts utilize the high frequencyoutput. Converting the high frequency output to DC with the desiredcharacteristics, in general, requires small components and is moreeconomical. However, either output voltage can be accommodated.

There are two general types of pin arrangements in fluorescent tubes.One type has one pin at each end and is run by applying the AC output ofthe ballast across those pins. To start the fluorescent tube, a highervoltage is initially applied across the tube until the discharge startsor some predetermined time has elapsed. Refer now to FIG. 6, whichillustrates one embodiment of a power adapter according to the presentinvention. Power adapter 70 is designed to adapt the output of a coldstart ballast of the type used to power two-pin fluorescent tubes. Alimiter 71 limits the starting voltage to prevent damage to AC/DCconverter 72 and presents a load to the ballast that mimics the currentdrawn when the corresponding fluorescent tube starts. AC/DC converter 72converts the AC generated by the ballast to a DC voltage that can beused by regulator 73 to generate a constant current power supply topower the LEDs.

The second type of pin arrangement consists of two pins at each end ofthe tube. Tubes of this type have some form of startup mode in which acurrent between the pins on the ends of the tube heats the gas orotherwise facilitates the initial gas breakdown prior to applying thedischarge maintenance potential across one pin on each end of the tube.In this arrangement, the power adapter must mimic a tube that isstarting up in terms of the power drain and loads across the pins ateach end. Once the ballast perceives that the fluorescent tube hasstarted, the two pins on each end of the fluorescent tube are drivenwith the same AC voltage to maintain the discharge. In this mode, thepower adapter needs to rectify the supplied power in a manner to thatanalogous to that discussed above. Refer now to FIG. 7, whichillustrates another embodiment of a power adapter according to thepresent invention. Power adapter 80 operates with a 4-pin ballast thatsupplies separate power to the fluorescent tube's starter circuitry. Thetwo pins 81 from one fluorescent tube connector power a starter emulator84 that presents a load that mimics the starter circuit in thefluorescent tube for which the ballast was designed. Similarly, the twopins 82 from the other fluorescent tube connector power a second starteremulator 85. Once the ballast supplies the fluorescent tube drivingvoltage on one pin from the set of pins shown at 81 and one pin from theset of pins shown at 82, AC/DC converter 86 rectifies this AC signal andprovides the DC power to regulator 73.

As noted above, once all of the tubes in a light fixture have beenreplaced with LED-based alternatives, it may be advantageous to replacethe old ballast with an AC/DC converter that is more power efficientthan the combination of the old ballast and the power adapters in theLED-based light sources. If the old ballast is merely replaced, theinput to the LED-based light sources will switch to a DC voltage that istypically much less than the amplitude of the AC voltages supplied bythe ballast. In the case of a 4-wire output ballast, both pins in set 81will be connected together, and both pins in set 82 will also beconnected together. In one aspect of the present invention, the AC/DCconverters used in power adapters 70 and 80 include a sensor thatdetermines if the input to those converters is AC or DC. If the input isDC, the converter merely connects the DC lines to the power regulator.It should be noted the startup circuit emulators will, in general, notbe activated by the lower DC voltages supplied by the new ballast.

In the embodiments discussed above with reference to FIG. 4, one of theheat-dissipating surfaces was a magnet that provided an attachmentmechanism and a heat path to a surface of the enclosure in which thelight source is mounted. It should be noted that it is sufficient forstructure 59 shown in FIG. 4 to be ferromagnetic to provide the benefitsdiscussed above. Refer now to FIG. 8, which is a cross-sectional view ofanother embodiment of a light source according to the present invention.Light source 90 includes a heat-dissipating structure 91 that is inthermal communication with the LEDs. Structure 91 includes aferromagnetic material such as iron or nickel. A separate magnet 92 isinterposed between surface 95 and structure 91. Hence, structure 91 doesnot need to be a magnet; it is sufficient for structure 91 to havesufficient ferromagnetic material to assure that structure 91 binds tomagnet 92 with sufficient force to hold light source 90 in place.

In the above-described embodiments, the light sources utilize individualLEDs that are distributed on the surface of the heat-dissipatingstructure. However, small clusters of LEDs could also be utilizedprovided the local heat generated is less than the power that wouldcause the area to increase in temperature to an unacceptable level.

The above-described embodiments of the present invention have beenprovided to illustrate various aspects of the invention. However, it isto be understood that different aspects of the present invention thatare shown in different specific embodiments can be combined to provideother embodiments of the present invention. In addition, variousmodifications to the present invention will become apparent from theforegoing description and accompanying drawings. Accordingly, thepresent invention is to be limited solely by the scope of the followingclaims.

1-14. (canceled)
 15. A light source comprising: a substrate having afirst surface; and a plurality of light emitting diodes (LEDs) mountedon the first surface of the substrate, with the plurality of LEDs beingspaced apart from each other, such that the plurality of LEDs areconfigured to generate less than 2 watts per second of heat on anysquare inch of the first surface of the substrate during operation ofthe plurality of LEDs.
 16. The light source of claim 15, wherein thefirst surface of the substrate is a metallic surface that is in contactwith air having an ambient temperature.
 17. The light source of claim16, wherein the plurality of LEDs are each spaced a distance apart fromeach other sufficient to maintain an operating temperature of less than75° C. above the ambient temperature of the air during operation of theplurality of LEDs.
 18. The light source of claim 15, wherein theplurality of LEDs are spaced apart from each other such that theplurality of LEDs are configured to generate less than 1 watts persecond of heat on any square inch of the first surface of the substrateduring operation of the plurality of LEDs.
 19. The light source of claim15, wherein the plurality of LEDs are mounted directly on the firstsurface of the substrate.
 20. The light source of claim 16, furthercomprising an additional metallic surface disposed on a second surfaceof the substrate opposite the first surface and in thermal communicationwith the metallic surface on the first surface, such that the additionalmetallic surface is configured to dissipate additional heat from themetallic surface on the first surface.
 21. The light source of claim 20,wherein the additional metallic surface comprises a magnetizedferromagnetic material.
 22. The light source of claim 20, furthercomprising an insulating layer disposed between the respective metallicsurfaces.
 23. The light source of claim 22, further comprising at leastone via extending through the insulating layer to thermally couple therespective metallic surfaces.
 24. The light source of claim 15, whereinthe plurality of LEDs are configured to emit light from the light sourcein a direction normal to the first surface of the substrate duringoperation of the plurality of LEDs.
 25. The light source of claim 15,further comprising: first and second plug adapters configured to matewith a fluorescent tube connector; and a power adapter configured toconvert power from a fluorescent tube ballast presented on the first andsecond plug adapters to DC power that powers the plurality of LEDs. 26.The light source of claim 25, wherein the fluorescent tube ballast isconfigured to provide power in a first mode that starts a conventionalfluorescent tube of a predetermined type and switch to provide power ina second mode that powers the conventional tube in a maintenance phaseof operation.
 27. The light source of claim 26, wherein the poweradapter is configured to present a load to the fluorescent tube ballastin the first mode, with the presented load mimicking a load that wouldbe provided by the fluorescent tube when said fluorescent tube commencesoperation.
 28. A light source comprising: a substrate having first andsecond opposing metallic surfaces in thermal communication with eachother; and a plurality of light emitting diodes (LEDs) mounted on thefirst metallic surface of the substrate, with the plurality of LEDsbeing spaced apart from each other, such that the plurality of LEDs areconfigured to generate less than 2 watts per second of heat on anysquare inch of the first metallic surface during operation of theplurality of LEDs.
 29. The light source of claim 28, wherein the firstmetallic surface is in contact with air having an ambient temperatureand the plurality of LEDs are each spaced a distance apart from eachother sufficient to maintain an operating temperature of less than 75°C. above the ambient temperature of the air during operation of theplurality of LEDs.
 30. The light source of claim 28, wherein theplurality of LEDs are spaced apart from each other such that theplurality of LEDs are configured to generate less than 1 watts persecond of heat on any square inch of the first metallic surface duringoperation of the plurality of LEDs.
 31. The light source of claim 28,wherein the plurality of LEDs are mounted directly on the first metallicsurface of the substrate.
 32. The light source of claim 28, wherein thesecond metallic surface comprises a magnetized ferromagnetic material.33. The light source of claim 28, further comprising: an insulatinglayer disposed between the first and second metallic surfaces; and atleast one via extending through the insulating layer to thermally couplethe first and second metallic surfaces.
 34. The light source of claim28, wherein the plurality of LEDs are configured to emit light from thelight source in a direction normal to the first metallic surface of thesubstrate during operation of the plurality of LEDs.
 35. The lightsource of claim 28, further comprising: first and second plug adaptersconfigured to mate with a fluorescent tube connector; and a poweradapter configured to convert power from a fluorescent tube ballastpresented on the first and second plug adapters to DC power that powersthe plurality of LEDs.
 36. The light source of claim 35, wherein thefluorescent tube ballast is configured to provide power in a first modethat starts a conventional fluorescent tube of a predetermined type andswitch to provide power in a second mode that powers the conventionaltube in a maintenance phase of operation.
 37. The light source of claim36, wherein the power adapter is configured to present a load to thefluorescent tube ballast in the first mode, with the presented loadmimicking a load that would be provided by the fluorescent tube whensaid fluorescent tube commences operation.